The stratosphere, the layer of the atmosphere from about 10 to 50 kilometers above Earth’s surface, over the polar regions is normally very cold during winter, with strong westerly winds forming the polar vortex. However, on some occasions, the normally quiet vortex suddenly warms over a week or two, and the winds slow dramatically, resulting in easterly winds that are more similar to the summer climate. These events are known as “sudden stratospheric warmings” (SSWs). A recent article published in Reviews of Geophysics explores our current understanding of SSWs. Here, the authors explain the effects of SSWs on weather, the upper atmosphere, and space weather, and how these events may alter with climate change.
Where, why, and how often do sudden stratospheric warming events occur?
SSWs occur when vertically propagating atmospheric waves break in the stratosphere—rather like waves breaking on a beach—leading to the rapid slowdown of the westerly winds.
Atmospheric waves can originate in the troposphere (primarily from mountain ranges and land-sea contrasts but also from weather systems) or are generated internally in the stratosphere.
These planetary-scale waves can only propagate in westerly winds, so their influence on the stratosphere is limited to the winter hemisphere when the polar vortex is present.
Nearly all SSWs occur in the Northern Hemisphere because the planetary-scale waves have larger amplitudes—mainly because the mountain ranges are bigger and there is more contrast between oceans and land. SSWs are possible in the Southern Hemisphere—one SSW did occur in the Southern Hemisphere (in 2002), and a major weakening of the polar vortex occurred in 2019. On average SSWs occur about six times in a decade.
What are the weather impacts of SSWs?
The main observed impacts after SSWs in the Northern Hemisphere are found in the Euro-Atlantic region, where SSWs tend to lead to colder and drier weather in northern Europe, while southwestern Europe experiences higher amounts of rainfall due to the southward shift of the North Atlantic storm track. Snow is also more likely in southern England following SSWs. These changes are associated with the negative phase of the so-called North Atlantic Oscillation and can last for several weeks or even months after the SSW event. Impacts in other regions are less well documented, but changes in North American weather have also been suggested to be related to the stratosphere, especially cold air outbreaks.
Though SSWs in the Southern Hemisphere are rare, a weaker than normal Southern Hemisphere polar vortex is associated with an equatorward shift of the Southern Hemisphere jet stream, warmer and drier conditions over southeast Australia, and colder and wetter conditions over New Zealand and southern Chile.
How do SSWs affect the stratosphere and stratospheric chemistry/ozone?
SSWs disrupt the atmospheric circulation, and are associated with changes to the concentrations of ozone and other trace gases throughout the stratosphere.
After the onset of an SSW, ozone increases above about 24 km and decreases below that level. The region of increased ozone then slowly descends, and the region of ozone increases relaxes back to normal.
The enhanced poleward and downward transport during an SSW enhances transport of other species such as carbon monoxide (CO) and nitrogen oxides (NOx) as well, with the breakdown of the polar vortex enhancing mixing between mid and high latitudes.
This will lead to cutting short ozone depletion by halogens in the Arctic polar stratosphere during spring, whereas in undisturbed and very cold winters Arctic ozone loss is more likely to occur (such as in winter 2010/2011 that featured unprecedented Arctic ozone loss).
What are the effects of SSWs above the stratosphere and on space weather?
In the high latitude mesosphere, the temperature decreases by 10s of Kelvins, and the winds reverse from easterly to westerly (opposite of the changes in the stratosphere). SSWs have a pronounced effect on the near-Earth space environment. Strong changes occur in the low latitude ionosphere electron density, a level of change that is on par with what occurs during moderate strength geomagnetic storms.
SSWs may further influence the generation of small-scale, turbulent-like, structures in the ionosphere. These structures negatively impact satellite-based navigation (e.g., GPS) and communication signals. The drag experienced by low-Earth orbiting satellites is decreased during SSWs. Fully understanding the effects of SSWs on the upper atmosphere is therefore critical due to their effects on technological infrastructure that is increasingly relied upon by society.
What are some of the recent advances in our understanding of SSWs?
Correctly predicting when SSWs occur can lead to windows of opportunity for forecasting surface weather, including extreme events such as cold air outbreaks and rainfall extremes. However, there remain many open questions with regard to the exact changes in skill induced by the stratosphere; for example, if SSWs are poorly predicted, this can still lead to major forecast busts over Europe. Furthermore, large unknowns remain with respect to changes in prediction and projection related to the stratosphere in other regions of the globe, especially in the Southern Hemisphere.
An extensive body of research in recent years has demonstrated that the impact of SSWs extends throughout the whole atmosphere. This includes the extension downwards to influence surface weather, as well as upwards into the mesosphere, thermosphere, and ionosphere (~60–300 km).
How might SSW events alter as the climate continues to change?
This remains unclear despite many efforts to study it in recent decades.
Analysis of the most recent intercomparisons of climate models does not provide an answer. Most individual CMIP6 models project significant changes but with no consensus on the sign of the change. Model biases in the atmospheric response to climate change might explain part of this uncertainty.
The increase of greenhouse gases has opposing effects in the polar stratosphere (radiative cooling vs. increased adiabatic warming from enhanced wave activity) and so, a different relative strength of these effects among models can lead to a different sign of the SSWs change.
Models also differ in the sensitivity of the Arctic to greenhouse gas increases, as well as the tropospheric response to SSWs. A better knowledge of stratosphere-troposphere dynamics and a consequent improvement of models would clarify future changes in SSWs.
What are some of the unresolved questions where additional research, data or modeling is needed?
Relatively simple dynamical models are not able to capture all mechanisms involved in the occurrence of SSWs, preventing their use in studying aspects such as the predictability of SSWs. State-of-the-art numerical models are also problematic for studying SSW predictability due to their biases in the stratosphere, particularly in the lowermost polar stratosphere. To resolve these issues, the Stratospheric Network for the Assessment of Prediction (SNAP) project is seeking to characterize and compare stratospheric biases in sub-seasonal prediction models.
The tropospheric response to SSWs also accounts for a big uncertainty, since the troposphere is not always clearly influenced by individual SSW events.
Finally, the full extent to which SSWs impact the near-Earth space environment also remains unknown. SSWs seem to impact the generation of small-scale ionospheric disturbances, but the evidence is far from conclusive. Understanding these effects is of particular importance owing to their influence on communication and navigation signals.
―Mark P. Baldwin ([email protected], 0000-0002-6273-4128), University of Exeter, UK; Blanca Ayarzagüena (0000-0003-3959-5673), Universidad Complutense de Madrid, Spain; Thomas Birner (0000-0002-2966-3428), University of Munich, Germany; Neal Butchart (0000-0002-4993-7262), Met Office Hadley Centre, UK; Amy H. Butler (0000-0002-3632-0925), NOAA Chemical Sciences Laboratory, USA; Andrew J. Charlton‐Perez (0000-0001-8179-6220), University of Reading, UK; Daniela I. V. Domeisen (0000-0002-1463-929X), ETH Zurich, Switzerland; Chaim I. Garfinkel (0000-0001-7258-666X), The Hebrew University, Israel; Hella Garny (0000-0003-4960-2304), Institut für Physik der Atmosphäre, Germany; Edwin P. Gerber (0000-0002-6010-6638), New York University, USA; Michaela I. Hegglin (0000-0003-2820-9044), University of Reading, UK; Ulrike Langematz (0000-0002-5102-0592), Freie Universität Berlin, Germany; and Nicholas M. Pedatella (0000-0002-8878-5126), National Center for Atmospheric Research, USA